This invention relates generally to the hemming of sheet metal and more particularly, to a hemming apparatus and method for forming a generally flat hem.
It is well known to construct motor vehicle body panels, doors, hoods, fenders, tailgates, trunks and deck lids by stamping an outer sheet metal panel and separately stamping an inner sheet metal reinforcing panel and then joining the two panels together by hemming a flange of the periphery of the outer panel over an adjacent edge of the inner panel to secure the panels together. Desirably, the outer panel is slightly larger than the inner panel to provide a border flange portion along the periphery of the outer panel that can be folded over the peripheral edge of the inner panel to define the hem flange that connects the two panels.
When the flange is folded over the peripheral edge of the inner panel with a traditional hemming apparatus and method, the resulting edge of the hemmed panels has a generally smooth, curved or arcuate shape with a diameter of the bend equal to twice the thickness of the outer panel plus the thickness of the inner panel. With inner and outer panels of the same thickness, the diameter of the bend for a standard hemming apparatus and method is equal to three times the thickness of a sheet metal panel. While many standard hemming methods and devices produce a smooth and fair hem or fold line, the relatively large bend diameter of the fold line reflects light in various directions along the curved fold line. This creates the visual impression that the gap between adjacent hemmed panels of the vehicle body is larger than it actually is, that the gap is inconsistent or that the panels are not flush with one another.
According to known hemming processes, an outer sheet metal panel is stamped and formed to include a border flange along a periphery of the outer panel. An inner sheet metal reinforcing panel is formed and shaped to be slightly smaller than the outer sheet metal panel. The inner panel is placed against the outer panel such that a periphery of the inner panel is disposed adjacent and generally parallel to the border flange of the outer panel. The two panels are then secured together by hemming the border flange of the outer panel over the adjacent periphery of the inner panel. In more conventional hemming processes this is done by folding the border flange over the inner panel such that the flange lies flat against the inner panelxe2x80x94resulting in a relatively large flange bend diameter. In a less-conventional process known as an xe2x80x9copen hemxe2x80x9d process, the panels are secured together by folding the border flange over the inner panel such that the flange is inclined against the outer edge of the inner panel and an outer edge of the flange is spaced from the inner panel. This leaves a smaller flange bend diameter but also leaves an elongated gap between and along the outer edge of the flange and the inner panel that foreign material can collect in.
The invention is a sheet metal hemming apparatus for securing two sheet metal panels together by forming a generally flat hem along respective peripheries of the two panels. The apparatus includes a fixture configured to receive and hold a sheet metal panel having an upstanding border flange and a hemming tool movably supported adjacent the fixture and configured to bend the upstanding border flange of a sheet metal panel supported on the fixture into a position forming an acute angle relative to an underlying portion of the panel. The hemming tool is configured to bend an outer portion of the flange into a position over and generally parallel to the underlying portion of the sheet metal panel.
The invention also includes a method for forming a generally flat hem along respective peripheries of the two panels. According to this method one can secure two sheet metal panels together by forming a generally flat hem along respective peripheries of the two panels. An outer sheet metal panel is formed to include a generally upstanding border flange along a periphery of the outer panel. The border flange is then folded until the flange is inclined over an underlying portion of the outer panel. The border flange is then further folded such that an outer portion of the flange is disposed generally parallel to an underlying portion of the outer panel.
Objects, features and advantages of this invention include providing a relatively simple hemming apparatus that produces a hemmed flange having a first portion inclined relative to a second portion, that provides a hemmed flange with a reduced bend diameter, that provides a fold line having a more consistent appearance when viewed from different angles, that provides adjacent hemmed panels on a vehicle visually appearing to have a narrower or smaller gap between them, that eliminates the need for a separate assembly operation to provide sealing material between the hemmed flange and inner panel by providing a closed hem, that is readily adaptable for use with panels having different configurations, that is reliable, durable, of relatively simple design and economical manufacture and assembly and that has a long, useful service life.